I hold the bird that sings no more, Yet warriors clash behind my door. Prizefighters test their might, In my squared ring, day or night. What am I?

Ahoy, matey! I stand tall on land or sea, A pirate's colors fly high on me. What am I?

A silver serpent crosses the land. Drawing miles of straight lines, sometimes a bend. It never sleeps but lays on a bed. Like stairs taking people up, yet it always stays flat.

Like Rodin's 'The Thinker,' Perched on a pot, When lost in deep thought, I'm the best spot to squat

You mostly know me in my synthetized and comercialized form, but but you should know me as an animal useful for filtering the sea - just like you know that I can clean your skin or your countertop.